Mercury goes with the flow: Pacific-caught tuna is one of the major sources of mercury in the U.S. diet, and so scientists are trying to understand how tuna pick up mercury. Mercury levels in the eastern North Pacific (where a significant fraction of albacore tuna are caught) have been rising recently, even though that part of the ocean lacks deep-water sources (like undersea volcanoes) and little mercury is deposited directly on the surface. Recent measurements and computer modeling might have found the source: coal-fired power plants in Asia. The plants emit the toxic element, which then deposits in the western Pacific, and then is carried north and east by ocean currents. A complex chain of events brings the mercury and tuna together: after mercury-containing particles settle on the ocean, they stick to surface algae. When the algae die, they sink and are decomposed by bacteria that convert the mercury into far more toxic methylmercury. This happens at depths frequented by tuna (200 to 1,000 meters), providing a chance for the fish to absorb the toxin. (Environmental Health Perspectives)